Conventionally, a step-down DC/DC (direct current to direct current) converter includes a capacitor between an input thereof and ground to input a direct voltage into the input.
FIG. 1 shows a circuit configuration of a typical conventional step-down DC/DC converter. Referring to FIG. 1, a capacitor GC1 is provided between an input VIN and ground. Accordingly, an input voltage is equal to a voltage of the capacitor GC1. When the input voltage is input into the input VIN, the conventional step-down DC/DC converter outputs a predetermined output voltage that is lower than the input voltage.
In the conventional step-down DC/DC converter shown in FIG. 1, the input voltage, which is the same as the voltage of the capacitor GC1 when electrical charge is fully stored in the capacitor, is higher than the output voltage to operate the DC/DC converter properly. In other words, in order to output the predetermined output voltage, the input voltage is higher than the predetermined output voltage. Moreover, to output the output voltage stably, the input voltage is required to be sufficiently higher than the output voltage.
In such a configuration, if the input voltage becomes close to the output voltage due to some accident such as a power outage, the output voltage output from the step-down DC/DC converter becomes unstable. In many cases, the output VOUT is connected to some electrical device including a CPU of a computer or the like. Accordingly, the DC/DC converter that cannot supply the output voltage when the input voltage is lower than the output voltage is not preferable because such an accident could sometimes happen for some reasons.
The capacitor GC1 supplies electric charges to an input terminal IN of a controller 10 to prevent from the decrease of the input voltage. However, as the voltage of the capacitor GC1 decreases, the DC/DC converter cannot supply the output voltage.